This invention relates to coating compositions for abrasive backings and particularly to those compositions containing an aminoplast resin and a thermoplastic resin.
Coated abrasives generally comprise a flexible backing upon which a binder holds and supports a coating of abrasive grains. The backing can be selected from paper, cloth, film, vulcanized fiber, etc., or a combination of one or more of these materials. The abrasive grains can be formed of flint, garnet, aluminum oxide, alumina-zirconia, ceramic aluminum oxide, diamond, silicon carbide, and the like. Binders are commonly selected from phenolic resins, hide glue, urea-formaldehyde resins, urethane resins, epoxy resins, and varnish. Phenolic resins include those of the phenol-aldehyde type.
Coated abrasives may employ a make coat of resinous binder material in order to secure the abrasive grains to the backing, and a size coat of resinous binder material can be applied over the make coat and abrasive grains in order to more firmly bond the abrasive grains to the backing. The resinous material of the make and size coats may be the same material or may be different materials. A common resinous material used for both make and size coatings is generically referred to as phenolic resin. Phenolic resins are a class of materials made from the reaction of phenol with various aldehydes.
Phenolic resins are commonly used in coated abrasive articles because of their high adhesive strength to abrasive particles, durability, and high thermal stability. There are two types of phenolic resins, resole and novolac. Resole phenolic resins have a molar ratio of formaldehyde to phenol greater than or equal to one to one, typically between 1.5:1.0 to 3.0:1.0. Novolac resins have a molar ratio of formaldehyde to phenol less than one to one.
The phenolic resins contain about 70 percent to about 85 percent solids, and preferably contain about 72 percent to about 82 percent solids. If the percent solids is very low, then more energy is required to remove the water and/or solvent. If the percent solids is very high, then the viscosity of the resulting phenolic resin is too high which leads to processing problems. The remainder of the phenolic resin is preferably water with substantially no organic solvent due to environmental concerns with the manufacturing of abrasive articles. Examples of commercially available phenolic resins include those known under the trade designations VARCUM and DUREZ, available from Occidental Chemical Corp., Tonawanda, N.Y.; AROFENE and AROTAP, available from Ashland Chemical Company, Columbus, Ohio; RESINOX, available from Monsanto, St. Louis, Mo.; and BAKELITE, available from Union Carbide, Danbury, Conn.
Although phenolic resins are widely used in the coated abrasives industry, phenolic resins do not adhere well to some types of backing materials. Poor adhesion may cause the phenolic binder to peel away or shear off prematurely as the abrasive article is subjected to normal use. This lack of adhesion limits the types of backings that can be used in coated abrasive articles that use phenolic resin binders.